Vinyl-Functional Silicones

Addition-cure chemistry provides an extremely flexible basis for formulating silicone elastomers. An important feature of the cure system is that no byproducts are formed, allowing fabrication of parts with good dimensional stability. Cures below 50 °C—Room Temperature Vulcanizing (RTV), cures between 50 and 130 °C—Low Temperature Vulcanizing (LTV), and cures above 130 °C—High Temperature Vulcanizing (HTV) are all readily achieved by addition-cure. The rheology of the systems can also be varied widely, ranging from dip-cures to liquid injection molding (LIM) and conventional heat-cure rubber (HCR) processing. Vinyl-terminated polydimethylsiloxanes with viscosities > 200 cSt generally have < 2% volatiles and form the base polymers for these systems. More typically, base polymers range from 1,000 to 60,000 cSt. The crosslinking polymer is generally a methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mole % methylhydrosiloxane. The catalyst is usually a complex of platinum in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts. By convention, the A part typically has the vinyl-containing silicone and the platinum catalyst at a level of 5-10 ppm, and the B part usually contains the hydride-functional siloxane.

Formulation of addition cure silicones must address the following issues:

• Strength – Unfilled silicones have extremely poor mechanical properties and crumble under pressure from a fingernail. The most effective reinforcing filler is hexamethyldisilazane-treated fumed silica. Alternatively, if clarity must be maintained, vinyl “Q” reinforcing resins are used.
• Hardness – Higher crosslink density provides higher durometer elastomers. Gels are weakly crosslinked systems and even contain substantial quantities of “free” fluids. In principle, molar equivalents of hydrides react with vinyls. See the section on hydride-functional fluids for further information. Also, polymers with vinyl pendant on the chain rather than at chain ends are utilized to modify hardness and compression set.
• Consistency – The viscosity of the base polymer and a variety of low surface area fillers ranging from calcium carbonate to precipitated silica are used to control the flow characteristics of silicone elastomers.
• Temperature of cure – Selection of platinum catalysts generally controls the preferred temperature of cure. (J Inorg Organomet Polym 6, 123–144, 1996; J Mol Cat. A: Chem 104, 293, 1996) Platinum in vinyldisiloxanes is usually used in room temperature cures. Platinum in cyclic vinylsiloxanes is usually used in high temperature cures.
• Work time (speed of cure) – Apart from temperature, moderators (sometimes called retarders) and inhibitors are used to control work time. Moderators slow but do not stop platinum catalysts. A typical moderator is tetravinyltetramethylcyclotetrasiloxane. Inhibitors stop or “shut-down” platinum catalysts and therefore are fugitive, i.e volatile or decomposed by heat or light (UV). Acetylenic alcohols such as methylisobutynol are volatile inhibitors. Patent literature shows that t-butylhydroperoxide is an effective inhibitor that breaks down at temperatures > 130 °C.
• Low temperature properties/optical properties – The introduction of vinyl polymers with phenyl groups alters physical properties of elastomers. At levels of 3-4 mole %, phenyl groups improve low temperature properties. At higher levels, they are used to alter the refractive index of elastomers, ranging from matching fillers for transparency to optical fiber applications. Unfortunately, increased phenyl substitution lowers mechanical properties of elastomers.
• Shelf-life – A fully compounded elastomer is a complex system. Shelf-life can be affected by moisture, differential adsorption of reactive components by fillers, and inhibitory effects of trace impurities. Empirical adjustments of catalyst and hydride levels are made to compensate for these effects.
• Compounding – All but the lowest consistency elastomers are typically compounded in sigma-blade mixers, planetary mixers, two-roll mills or, for large scale production, twin-screw extruders.

Quick Start Formulation Transfer and Impression Molding Elastomer: This low strength formulation is useful as a reproductive molding compound. It is presented here because it can be prepared without special equipment and is an instructive starting point for addition cure silicone elastomers.

Activated-cure silicone elastomers are processed by methods consistent with conventional rubbers. These silicone products are referred to as HCRs (heat-cured rubbers). The base stocks are high molecular weight linear polydiorganosiloxanes that can be converted from a highly viscous plastic state into a predominantly elastic state by crosslinking. Vinylmethylsiloxane-dimethylsiloxane copolymers of extremely high molecular weights are the typical base stocks for activated-cure silicone elastomers. The base stocks are commonly referred to as gums. Gums typically have molecular weights from 500,000 to 900,000 with viscosities > 2,000,000 cSt. The silicone rubbers derived from the gums by compounding reinforcing agents extenders and additives are divided into three main classes: VMQ (dimethyl silicone/regular silicone), PVMQ (diphenyl dimethyl silicone/low temperature silicone) & FVMQ (fluorosilicone/fuel-resistant silicone).

Free-radical coupling (cure) of vinyl and methyl groups is usually initiated by peroxides at process temperatures of 140-160 °C. Generally, peroxide loading is 0.2-1.0%. Following the cure, a post-cure at 25-30 °C higher temperature removes volatile peroxide decomposition products and stabilizes polymer properties. The most widely used peroxides include dibenzoylperoxide (often as a 50% concentrate in silicone oil), dicumylperoxide, (often 40% on calcium carbonate), 2,5-dimethyl-2,5-di-t-butylperoxyhexane, and bis(dichlorobenzoyl)peroxide.^1,2 The last peroxide is particularly recommended for aromatic-containing siloxanes. Terpolymer gums containing low levels of phenyl are used in low-temperature applications. At increased phenyl concentrations, they are used in high temperature and radiation-resistant applications and are typically compounded with stabilizing fillers such as iron oxide. Phenyl groups reduce crosslinking efficiency of peroxide systems and result in rubbers with lower elasticity. Fluorosilicone materials offer solvent resistance. Lower molecular weight vinylsiloxanes are frequently added to modify processability of base stocks.

¹ Lynch, W., “Handbook of Silicone Rubber Fabrication”, Van Nostrand Reichold, 1978.
² Brassard, D.M., “The Silicone Elastomer Handbook”, Silicone Solutions, 2010.

While the use of peroxide-activated cure chemistry for vinylmethylsiloxanes is well established for gum rubber stocks, its use is growing in new applications that are comparable to some peroxide-cure acrylic systems. Relatively low viscosity vinylmethylsiloxanes and vinyl T-fluids are employed as grafting additives to EPDM elastomers in the wire and cable industry to improve electrical properties. They also form reactive internal lubricants for vulcanizable rubber formulations. At low levels they are copolymerized with vinyl monomers to form surfactants for organosols.

Vinyl-containing copolymers are used as crosslinkers in Pt- and peroxide-cure elastomer. High vinyl content copolymers form elastomers used in high accuracy soft lithography.^1,2,3

1. Choi, D. et al. Mat. Sci. Eng. C. 2004, 24, 213.
2. Infuehr, R. et al. Appl. Surf. Sci. 2003, 254, 836.
3. Schmid, H.; Michel. B. Macromolecules 2000, 33, 3042.

See also Hydride Q resins.